Cryptographic technology in information transmission has been studied in response to recent development of information-communication technologies such as electronic commerce and electronic mails. Quantum cryptography has been drawing much attention recently as the cryptographic technology.
In the quantum cryptography, security of communication is guaranteed by a physical phenomenon based on Heisenberg uncertainty principle in quantum mechanics. In the uncertainty principle, a quantum state is changed by observation, and therefore wiretapping (observation) of communication is inevitably detectable. This allows to take measures against the wiretapping, such as shutting down the communication, thus, the wiretapping is considered to be physically impossible. Additionally, it is impossible to replicate particles due to the uncertainty principle.
Quantum teleportation is one of the key features of the quantum cryptography. The quantum teleportation is a technique for transmitting only quantum information of the particles. The quantum teleportation is achieved by information exchanges between photons by utilizing a quantum entangled state. A photon pair in the quantum entangled state has such a property that a quantum state of one of the photons is determined when that of the other is determined. This property is not dependent on a distance between the two photons.
In order to achieve the quantum teleportation, it is required to produce the photon pairs in the quantum entangled state. The photon pairs in the quantum entangled state can be produced by parametric down-conversion that is one of the second-order nonlinear optical processes.
As a technique for producing the quantum entangled photon pairs by using the parametric down-conversion, Non-Patent Citation 1 discloses a technique for spontaneously producing quantum entangled photon pair from a crystal, the photon pair being composed of photons whose polarization directions are perpendicular to each other, based on type II phase matching condition. In addition, Non-Patent Citation 2 discloses a technique for producing a quantum entangled photon pair in two nonlinear optical crystals superimposed on each other and oriented at 90° with each other, by using the parametric down-conversion based on type I phase matching condition, the photon pair being composed of photons whose polarization directions are parallel to each other.
Further, a quantum entangled photon pair producing device utilizing a Mach Zehnder interferometer has been known as a technique for producing quantum entangled photon pairs by using photon pairs outputted from nonlinear optical crystals in an axial direction of pump light (see Non-Patent Citations 3 and 4). The quantum entangled photon pair producing devices described above produce the quantum entangled photon pairs by combining, by means of a polarization beam splitter provided on an output side of the interferometer, the photon pairs outputted respectively from the two nonlinear optical crystals.
[Non-Patent Citation 1]
    “New high-intensity source of polarization-entangled photon pairs”, P. G. Kwiat, et al., Phys. Rev. Lett. 75, 4337 (1995).[Non-Patent Citation 2]    “Ultrabright source of polarization-entangled photons”, P. G. Kwiat, et al., Phys. Rev. A 60, R773 (1999).[Non-Patent Citation 3]    “Interferometric Bell-state preparation using femtosecond-pulse pumped spontaneous parametric down-conversion”, Yoon-Ho Kim, et al., Phys. Rev. A 63, 062301 (2001).[Non-Patent Citation 4]    “Generation of ultrabright tunable polarization entanglement without spatial, spectral, or temporal constrains”, M. Fiorentino, et al., Phys. Rev. A 69, 041801 (R) (2004).